1. Field of the Invention
The invention relates to a gas detection instrument having a combustible gas sensor, which requires power to heat one or two sensor elements in order to generate a signal corresponding to gas concentration.
2. Description of Related Art
Combustible gases present a significant hazard in the work place. At the present time, there are mainly three sensor technologies available for detecting combustible gases, the pellistor or catalytic bead sensor, the metal oxide semiconductor sensor, and the infrared sensor. Among the three technologies, the catalytic bead and semiconductor sensors are most widely used in the safety market due to their low costs and compact sizes.
Catalytic bead sensors started out as hot-wire gas detectors, in which the change in resistance of a heated platinum filament due to combustion of the gas on the surface of the wire provided the detection signal. Greatly improved performance, especially with respect to poisoning, was achieved by forming a porous bead of alumina (or other refractory material) around the coil, and then applying a precious metal catalyst to the surface, and within the bulk of the alumina bead. This catalytic bead sensor operates by heating the bead to about 500-600° C. by internal electrical heating, and then measuring small changes in the electrical resistance of the bead in the presence of a combustible gas due to the increase in temperature caused by gas combustion.
This small resistance change is typically measured using a Wheatstone bridge circuit. The majority of catalytic bead sensors have two beads, a catalytic bead and a reference bead which is not catalytically active but which is used to compensate for changes in the electrical resistance of the catalytic bead due to changes in ambient conditions.
Metal oxide semiconductor (MOS) sensors are long-lived, and more sensitive to combustible gases. A MOS sensor typically comprises a sintered metal (tin, zinc, or iron) oxide film on an alumina or silica substrate, which surrounds a heater coil. Two noble metal electrodes are applied to the oxide-coated substrate. This entire sensing element is enclosed in a metal housing with a stainless steel mesh cover acting as both a flame arrestor and a gas entry port. During operation, the sensing element is heated to about 250-350° C. When gas enters the sensor, it interacts with the oxide coating, causing a decrease in resistance between the two electrodes. The output of the MOS sensor varies logarithmically with the gas concentration. The accuracy, repeatability, and stability of MOS sensors are poor compared with catalytic bead sensors.
Combustible gas sensors require power to operate. As has been noted, the sensing elements in these sensors must be heated to a certain temperature in order to detect gas, and the sensors thus have a power consumption which is typically above 200 mW. This power requirement has been a major concern when designing a portable gas detection instrument, where available battery power determines the run time of the instrument.
In order to reduce power consumption, electricity may be supplied to the sensor at given intervals, as proposed in Japanese Provisional Utility Model Publication No. 14959 of 1987. This intermittent operation saves about 30-60% power, while at the same time, allows gas detection. This sensor operation, however, has a major drawback. The duration of electricity supply in each interval must be sufficiently long for the sensing element of the sensor to be heated up to desired temperature to detect gas; otherwise sensor sensitivity and accuracy will be affected. The intervals of power supply are, therefore, generally limited to at least a few seconds, which means a long waiting period for the instrument to detect, and update visual display and activate an audible alarm in the presence of gas.
U.S. Pat. No. 6,348,872 discloses a gas detector which comprises a hot-wire sensor and an actuating power control that supplies pulsed actuating power with two pulse cycles. When the gas concentration detected by the sensor is low a long cycle is used; when the gas concentration exceeds a prescribed level a shorter pulse cycle is used. This instrument has about the same limitations as other instruments with intermittent power supply due to the minimum requirement for duration of each cycle.
Japanese Provisional Patent Publication 03-233699 describes a combustible gas detector in which the sensor is operated with intermittent power, but the power is changed to continuous power in the presence of gas. The instrument continues to supply power as long as gas is present. Although the instrument is capable of detecting transient changes in gas concentration and is especially good for monitoring the occurrence of gas leakage, continued heating of sensing elements substantially increases power consumption in the presence of gas.